Nomenclature and Isomerism
Organic chemistry foundations
Organic chemistry is the study of carbon compounds. Carbon forms four covalent bonds and chains/rings, giving millions of compounds. Molecules are grouped into homologous series — families with the same functional group and general formula, differing by CH₂.
Types of formula
- Empirical: simplest whole-number ratio of atoms.
- Molecular: actual number of each atom (e.g. C₄H₁₀).
- Structural / displayed / skeletal: show how atoms are arranged; skeletal shows bonds/carbon skeleton without labelling every C and H.
Nomenclature (IUPAC naming)
1. Stem = longest carbon chain (meth, eth, prop, but, pent, hex…).
2. Suffix/prefix = functional group (-ane, -ene, -ol, -al, -one, -oic acid…).
3. Locants = numbers showing where groups are (choose the lowest set).
4. Substituents listed alphabetically with di/tri prefixes for repeats.
Example: CH₃CH(CH₃)CH₂OH = 2-methylpropan-1-ol.
Functional groups (key ones)
| Group | Suffix/prefix | Example |
|---|---|---|
| Alkene (C=C) | -ene | ethene |
| Halogenoalkane | chloro-/bromo- | chloroethane |
| Alcohol (–OH) | -ol | ethanol |
| Aldehyde (–CHO) | -al | ethanal |
| Ketone (C=O) | -one | propanone |
| Carboxylic acid (–COOH) | -oic acid | ethanoic acid |
Isomerism
Isomers have the same molecular formula but different arrangements.
Structural isomers (same formula, different structure)
- Chain isomers (different carbon skeleton), position isomers (functional group in a different place), functional group isomers (different group entirely, e.g. an alcohol vs an ether).
Stereoisomers (same structure, different spatial arrangement)
- E–Z (cis–trans) isomerism: occurs around a C=C double bond (no rotation) when each carbon has two different groups. E (opposite sides), Z (same side).
- Optical isomerism: a chiral centre (a carbon with four different groups) gives two non-superimposable mirror-image molecules (enantiomers) that rotate plane-polarised light in opposite directions.
Worked example
Why does but-2-ene show E–Z isomerism but but-1-ene does not?
- E–Z needs a C=C with two different groups on each carbon. But-2-ene (CH₃–CH=CH–CH₃) satisfies this; but-1-ene has a CH₂= end with two identical H atoms, so no E–Z isomerism. ✓
Common mistakes
- Numbering the chain from the wrong end (use the lowest locants).
- Confusing structural isomerism (different connectivity) with stereoisomerism (same connectivity, different arrangement).
- Forgetting a chiral centre needs four different groups.
Exam tips
- Practise IUPAC naming: stem → functional group suffix → locants → substituents.
- Classify isomers correctly (chain/position/functional vs E–Z/optical).
- Identify chiral centres for optical isomerism.
Key facts to remember
- Homologous series share a functional group and general formula (differ by CH₂); learn empirical/molecular/structural/skeletal formulae.
- IUPAC name = stem + functional group + locants + substituents.
- Structural isomers (chain/position/functional group); stereoisomers: E–Z (around C=C) and optical (chiral centre = 4 different groups → enantiomers).